Silk screening is the process by which a screen or mesh is obliterated in negative image areas while leaving image areas of the screen open so that printing inks may pass through the screen in the open-mesh image areas to produce an image on a surface immediately behind the screen. To make a suitable printing screen, a mesh of silk, polyester, nylon or stainless steel wire is stretched across a frame, tautened and affixed to the frame to prevent the mesh from sagging or going limp during the printing process.
There have been, in the past, several known state of the art methods of non-photographic screens. For example, paper stencils are cut to create a negative image, then the paper is adhered to the screen, and printing ink is passed through the uncovered image areas to create the positive image on a surface therebehind. This method is useful for printing only a few impressions and cannot be used when fine detail is required. Blockout is a method used by applying a masking material to the desired negative image areas of a screen. The blockout material could consist of a glue, shellac or any polymer not affected by the screen printing ink. The old touche and glue method of making screens is sometimes being employed by those knowledgeable and skilled in this art, however, this is done to a limited extent, due to the fact fine detail is difficult to achieve and making a screen in this fashion is very laborious.
Another method of making a non-photographic stencil with hand-cut stencils. In this process, a film of gelatin-like layers are applied evenly over a paper or plastic film, the layers are hand-cut with a sharp blade to the image desired, and the gelatin is adhered to the mesh with a solvent or with water, depending upon the chemical composition of the gelatin. Once adhered and dried, the backing sheet is peeled from the screen, leaving the cut image to create the stencil.
A more recently developed process for imaging screens involves the use of photosensitive coatings, herein referred to as "emulsions" to coat the screen. The emulsions contain polymers which cross-link when exposed to actinic radiation sources, i.e., sources that produce light in the visible and/or ultraviolet frequencies. The photosensitive emulsions are coated on meshes or screens which are tautly stretched across a frame under darkened or safe-light conditions. The emulsions are allowed to dry, and an obliterating material, such as an opaque film positive, is placed over the screen for shielding imaging areas of the screen, and the screen is then exposed to an actinic light source which cross-links the polymers in the exposed, negative image areas of the screen. The cross-linked polymer bridges the mesh and thereby exhibits increased adherence to the mesh relative to the non-cross-linked emulsion in the imaging areas of the screen that were shielded by the obliterating material. The non-exposed emulsion is selectively removed from the screen, for example, by washing the screen with warm water. After the screen is thoroughly dried, the screen can be used for printing, ink passing through the image areas from where the non-exposed emulsion has been removed.
In a typical procedure, a screen printer simply applies the emulsion to the screen in a liquid state, allows it to dry, and then places the obliterating material over the dried polymer before exposing it to light. In a similar process, which is particularly suitable for producing excellent detail, the emulsion is applied to a sheet of transparent plastic. After the emulsion is dried, the sheet is laid over the screen with the emulsion against the screen. The emulsion is then re-solvated and redried. The obliterating material is laid directly over the sheet and the obliterating material side is exposed to light. Then the backing sheet is peeled away and the non-exposed emulsion is washed away from the image areas.
A variety of actinic radiation sources may be used to expose the screens, including carbon arc lamps, mercury vapor lamps, fluorescent lamps, particularly those with ultraviolet radiation-emitting phosphors, argon glow lamps, electronic flash lamps and photographic flood lamps. Both visible and ultraviolet light is suitable, but preferably the light is in or is weighted to the ultraviolet spectra.
The most commonly used type of photosensitive emulsion includes as the cross-linking polymers, poly vinyl alcohol and a copolymer of poly vinyl acetate. The poly vinyl acetetate copolymer has the general formula A.sub.x B.sub.1-x where A is vinyl acetate and B is the comonomer. A typical comonomer for vinyl acetate is ethylene. The polymers are emulsified in water, and other ingredients, such as defoamers, colorants and plasticizers may be added, as is known to those skilled in the art.
To make the emulsions photosensitive, free radical-generating compounds, such as ammonium dichromate, potassium dichromate or diazo compounds, such as copolymer formaldehyde/p-diazo diphenylamine sulfate or benzenediazonium, 4-(phenylamino)-sulfate 1:1 polymer with formaldehyde, are admixed with the emulsion. To prevent premature cross-linking, the free radical-generating sensitizing compound is generally admixed with the emulsion just prior to its application as a coating to a screen. Generally, a manufacturer of an emulsion will recommend a specific activating chemical to be admixed in a specific amount with a particular emulsion.
The relative amounts of the poly vinyl alcohol and the poly vinyl acetate copolymer is selected according to the solvent-resistance or water-resistance desirably exhibited by the emulsion subsequent to its cross-linking. If a solvent-based ink is to be used, solvent-resistance is desired, and if a water-based ink is to be used, water-resistance is desired. Generally, higher proportions of the poly vinyl alcohol increase the solvent-resistance of the cross-linked emulsion film, whereas higher proportions of the poly vinyl acetate copolymer increase the water-resistance of the cross-linked emulsion film. Emulsion films of this type may either be solvent-resistant or water-resistant, but not both unless further treated. This is unfortunate, as it is frequently desirable to use the same screen for printing with both water-based and with solvent-based inks, for example, if a logo is to be printed onto two different fabrics, one receptive to water-based ink and other receptive to solvent-based ink.
While light-exposed, cross-linked emulsion films of the type described above are not both water-resistant and solvent-resistant, hardening techniques are known which make the emulsion films resistant both to solvent and to water. Such post treatment hardening techniques include exposing the screen to harsh chemicals, such as hydrochloric acid. A chelated ester of orthotitanic acid, such as tetrabutyltitanate, is often used for hardening. Sometimes used for hardening, but less often, are mild solutions of sodium hydroxide or calcium hydroxide. Each of these hardening solutions stabilizes the emulsion films on the screen by increasing the degree of cross-linking.
An important disadvantage of using hardening chemicals is that the hardened films cannot be easily removed from the mesh, and therefore, the mesh cannot be reclaimed for producing other images. Considering that mesh material is expensive and that the screen stretching process involves expensive labor, printers are reluctant to harden screens.
The emulsions presently used generally have a solids content (weight percentage of polymerizing material relative to total weight, including water) of at most about 30 to 35% weight percent. It would be desirable to increase the solids content of the polymer, as this would permit a printer to coat screens having coarse meshes with fewer applications of the emulsion. Other desirable attributes of photosensitive emulsions include high light sensitivity in order to minimize light exposure time, the ability to develop fine detail, which is a function of the strength of the cross-linked, light-exposed polymer film, the film strength being generally a function of the amount of polymer cross-linking. The cross-linked emulsion film should be readily removable with available emulsion removers to permit reuse of the screen.